This invention relates generally to the arrangement and operation of capacitive probes for monitoring changes in the properties of dielectric material.
The use of capacitive probes to monitor curing of dielectric materials such as epoxy resins and other plastics is generally well known. Heretofore, such probes were embedded in structural parts in which they remained or were removed, including sample parts being co-processed with other parts, as for example parts being cured in solid rocket motor environments. Questions always arise as to how representative the co-processed sample part is, such as a "witness panel" generally poured from an early or late portion of a batch which is of a smaller size and may occupy a stagnant corner of the curing oven. To avoid the problems arising in such environments, the use of capacitive probes that are not embedded in the material sample being monitored would appear to be desirable.
The use of a three terminal type of capacitive probe to sense the condition of material in which the probe electrodes are not embedded, has already been proposed as disclosed for example in U.S. Pat. Nos. 3,774,238, 3,781,672, and 3,826,979 to Hardway, Jr., Maltby et al. and Steinmann, respectively. The probe electrode arrangement as disclosed in the Hardway, Jr., patent addresses the problem of large cable capacitance as an extraneous influence without regard to material properties at a known and controlled depth to which an electric field, between the electrodes of the probe, penetrate the material. According to the Maltby, et al., patent, a probe is provided with an electrode arrangement including a guard electrode driven to the same potential as the voltage applied to a conductive measuring electrode for shielding purposes. Use of a probe electrode arrangement and support to control electric field penetration depth within the material sample being monitored, is however foreign to the disclosure in the Maltby et al., patent. As to the Steinmann patent, it discloses a probe electrode arrangement providing a signal receiver output for switch controlled operation of equipment such as a motor vehicle windshield wiper. Such output control function of the probe circuit disclosed in the Steinmann patent, is designed to detect the presence of material at a controlled distance therefrom, but not for quantification of dielectric properties of the material sample being monitored.
Accordingly, it is an important object of the present invention to provide a capacitive probe arrangement having an electrode geometry and support through which electric field penetration depth may be controlled as a function of lift-off spacing to establish a predetermined sensitivity region within the material from which monitoring measurements of material properties is made.
An additional object associated with the foregoing object is to provide a capacitive probe arrangement having measurement receiving circuitry capable of being shielded from extraneous influences as well as being insensitive to lift-off spacing between the material sample and the probe electrodes.
By virtue of the foregoing objectives, the present invention has as another object the provision of more accurate measurement and quantification of dielectric constants of dielectric material without embedding therein the probe electrodes during material cure.